This application is related to co-pending U.S. patent applications entitled xe2x80x9cSYSTEM FOR LASER SHOCK PROCESSING OBJECTS TO PRODUCE ENHANCED STRESS DISTRIBUTION PROFILESxe2x80x9dand xe2x80x9cARTICLES HAVING IMPROVED RESIDUAL STRESS PROFILE CHARACTERISTICS PRODUCED BY LASER SHOCK PEENINGxe2x80x9d, filed concurrently herewith by the same inventors and assigned to the same assignee as the present application, the contents thereof being incorporated herein by reference hereto.
1. Field of the Invention
The present invention relates generally to laser shock peening techniques, and, more particularly, to processing methods employing various laser shock peening procedures to enhance the deep compressive residual stress characteristics induced by laser shock peening and to selectively modify in a controlled manner the compressive residual stress distribution profile developed in a processed workpiece, such as an airfoil.
2. Description of the Related Art
Laser shock processing has found use in applications involving the enhancement of certain structural features such as the leading and trailing edges of turbine engine compressor or other airfoils. Various strategies have focused upon finding adequate laser beam spot patterns to process the airfoil. However, little attention has been given to determining useful techniques that can provide desired shockwave groups and accompanying stress distribution profiles.
In a typical application, when a shockwave from a single laser irradiated spot on the surface of a material propagates into the material from the surface, the peak pressure is highest at the surface and then decreases (i.e., attenuates) with increasing depth into the material. If the peak pressure is high enough, namely, above the dynamic yield strength of the workpiece, the shockwave plastically deforms the material below the surface in an amount generally proportionate to the amount that the peak pressure is above the dynamic yield strength.
The plastic yielding gives rise to plastic strain in the material, which creates the compressive residual stresses desired by the process. As the peak pressure of the shockwave decreases with increasing depth below the surface, the amount of plastic strain also decreases. This factor limits the depth of the compressive residual stress that can be introduced into the workpiece.
Various processing methods are provided that employ laser shock peening procedures to enhance the deep compressive residual stress characteristics induced within a workpiece by laser shock peening, such as with the introduction of an asymmetrical or other selectively configured compressive residual stress distribution profile within the workpiece. One operation may involve processing an airfoil to develop an asymmetrical stress distribution profile through the thickness dimension of a thin section of the airfoil. The asymmetrical stress distribution profile will be selectively tailored to produce compressive residual stress properties within the airfoil that have desired behaviors and objectives, such as retarding crack propagation, inhibiting the growth of incipient flaws, strengthening the material at high fatigue locations, increasing the high cycle fatigue strength at specific location, providing a desired shape or curvature, and other such uses as typically understood in the art.
According to one processing method, the workpiece is simultaneously irradiated with a set of laser beams to form a corresponding set of adjacent non-overlapping laser shock peened surfaces. The spaced-apart relationship between the laser beam spots is chosen such that the respective shockwaves induced by laser shock peening will encounter one another as they propagate through the workpiece. The shockwaves will intersect at a location disposed generally between the laser shock peened surfaces.
In one form, the encountering shockwaves will interact in a manner generally exhibiting a constructive interference effect. In this manner, the respective deep compressive residual stress regions that extend from each of the adjacent non-overlapping laser shock peened surfaces will overlap and significantly increase the peak pressure experienced by the material in the vicinity of the shockwave intersection plane. Various laser spot beam patterns may be developed to produce selective arrangements of shockwave interaction locations.
According to another method, the workpiece is irradiated at opposing sides thereof at different times to form opposing laser shock peened surfaces. In this manner, the opposing time-staggered shockwaves induced by laser shock peening will meet at a location apart from a mid-plane of the workpiece, producing an asymmetrical compressive residual stress profile through a thickness dimension of the workpiece. The relative difference between the arrival times of the laser beams used to laser shock peen the opposing sides of the workpiece is chosen to facilitate control of the profile characteristics by selectively determining the interior location where the opposing shockwaves will encounter one another.
According to another processing method, the workpiece is irradiated simultaneously at opposing sides thereof using laser beams having different pulse lengths to form opposing laser shock peened surfaces. The use of such differential laser beam pulse lengths results in the development of opposing shockwaves induced by laser shock peening that attenuate at different rates as they propagate through the workpiece. This disparate attenuation in the shockwaves will produce compressive residual stress regions extending from the respective laser shock peened surfaces having a stress distribution profile that exhibits an asymmetry along a thickness dimension of the workpiece.
According to another processing method, the workpiece is irradiated simultaneously at opposing sides thereof to form a set of laterally offset laser shock peened surfaces. This lateral offset has the effect of creating an imbalance in the forces that are developed within the workpiece as the shockwaves induced by laser shock peening propagate through the workpiece. This force imbalance exerts a moment force on the material, tending to rotate it around an axis perpendicular to the displacement vector connecting the laterally offset laser shock peened surfaces, and lying in the nominal mid-thickness phase between the opposing laser-peened surfaces.
Additionally, the oppositely-directed shockwaves will interact in a generally asymmetrical manner relative to a mid-plane of the workpiece, producing a shockwave interaction zone generally centered about the mid-plane but exhibiting wing-type portions that extend toward opposite ones of the workpiece surfaces in an oblique manner relative to the mid-plane. A corresponding asymmetrical stress distribution profile will accompany this particular form of shockwave interaction associated with the simultaneous formation of laterally offset laser shock peened surfaces disposed at opposing sides of the workpiece.
The invention, in one form thereof, is directed to a method that involves laser shock peening an object to form at least one set of at least two simultaneously formed, non-overlapping adjacent laser shock peened surfaces.
In one form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object which is sufficient to enable the formation of at least two overlapping regions each having compressive residual stresses imparted by laser shock peening, wherein each region extends into the object from a respective laser shock peened surface.
In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object which is sufficient to enable at least two respective shockwaves induced by laser shock peening in connection with the simultaneous formation of at least two respective non-overlapping adjacent laser shock peened surfaces to encounter one another within the object.
In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object, which is configured to effectuate the formation of at least one row of spaced-apart shockwave intersection sites in the object, wherein each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from neighboring spaced-apart laser beam spots.
In another form, the laser shock peening step includes comprises the step of forming a selective laser beam spot pattern on the object including at least one row of laser beam spots arranged in spaced-apart overlapping pairs. The spatial relationship between adjacent pairs is sufficient to enable the formation of a shockwave intersection site disposed at least in part therebetween, wherein each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from nearest neighbor laser beam spots of adjacent laser beam spot pairs.
In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object including at least one row of non-overlapping laser beam spots configured to define a selective pattern of shockwave intersection sites. Each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from neighboring laser beam spots.
In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object including at least one row of overlapping laser beam spots, wherein the spot pattern is configured to effectuate the formation of at least one row of spaced-apart shockwave intersection sites in the object. Each row of shockwave intersection sites is generally disposed between respective adjacent ones of the laser beam spot rows, while each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from laser beam spots of adjacent rows.
In another form, the laser shock peening step further includes the step of sequentially forming at least one selective laser beam spot pattern on the object, wherein each pattern is configured to effectuate the formation of at least one row of spaced-apart shockwave intersection sites in the object. Each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from neighboring laser beam spots. Preferably, each row of spaced-apart shockwave intersection sites associated with a respective laser beam spot pattern has a respective orientation characteristic defining a directional orientation of the shockwave intersection sites associated therewith.
The object preferably includes a gas turbine engine component such as an airfoil.
The invention, in another form thereof, is directed to a method that involves laser shock peening an object to form at least one set of at least two non-overlapping adjacent laser shock peened surfaces simultaneously formed with one another. Each laser shock peened surface is associated with a respective shockwave induced by laser shock peening. Moreover, the respective shockwaves which are associated with at least one selective set of at least two simultaneously formed, non-overlapping adjacent laser shock peened surfaces encounter one another within the object.
The invention, in another form thereof, is directed to a method that involves simultaneously laser shock peening an object at a plurality of locations to form at least one pair of adjacent, spaced-apart laser shock peened surfaces on the object and to induce the generation of a respective shockwave in association with the formation of each laser shock peened surface. The respective spaced-apart relationship between the respective laser shock peened surfaces of at least one respective laser shock peened surface pair is sufficient to enable the respective shockwaves associated therewith to encounter one another within the object.
The invention, in another form thereof, is directed to a method that involves laser shock peening an object to form at least one set of at least two simultaneously formed, spaced-apart adjacent laser shock peened surfaces. Each laser shock peened surface is associated with a region of compressive residual stresses extending into the object therefrom and imparted by laser shock peening. The laser shock peening operation is configured to enable the formation of at least one region overlap location, wherein each region overlap location defines a respective overlap of at least two respective compressive residual stress regions respectively associated with at least two corresponding simultaneously formed, spaced-apart adjacent laser shock peened surfaces.
The configuration step further includes the step of selecting a predetermined spaced-apart relationship for use in forming neighboring ones of the laser shock peened surfaces.
The invention, in another form thereof, is directed to a method that involves providing a laser shock processor; and operating the laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of at least two shockwaves having mutually non-interfering initial wavefronts to develop simultaneously at a selective side of the object and subsequently interact with one another within the object.
In one form, the laser shock processor is operated to simultaneously form two spaced-apart laser shock processed surfaces on the object.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form at least one set of an associated first laser shock peened surface and a second laser shock peened surface on the first side and the second side of the object, respectively. The associated first and second laser shock peened surfaces of each respective set of laser shock peened surfaces are formed at different times. Preferably occurring between 1 nanosecond and 2000 nanoseconds apart in time.
In a preferred form, the associated first and second laser shock peened surfaces of at least one respective set of laser shock peened surfaces are disposed generally opposite one another.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves forming at least one pair of associated laser shock peened surfaces each disposed at a different one of the first and second sides of the object. Each pair of associated laser shock peened surfaces is formed by laser shock peening the object at the first and second sides thereof at different times.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of generally opposing shockwaves to develop in a time-staggered relationship to one another at different ones of the first and second sides of the object.
In a preferred form, the laser shock processor is operated to form a first laser shock peened surface on one of the first and second sides of the object; and form a second laser shock peened surface on the other of the first and second sides of the object, at a time later than the formation of the first laser shock peened surface.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of generally opposing shockwaves to develop at different times at different ones of the first and second sides of the object and to subsequently encounter one another within the object at a location apart from a mid-plane of the object.
In a preferred form, the laser shock processor is operated to laser shock peen the object at the first and second sides thereof at different times to form generally opposing laser shock peened surfaces.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause the formation of at least one set of generally opposing regions each extending from respective laser shock peened surfaces formed at different times at different ones of the first and second sides of the object. Each region has compressive residual stresses imparted by laser shock processing. Furthermore, each set of opposing regions defines an asymmetrical compressive residual stress distribution profile appearing generally along a respective thickness dimension of the object.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves simultaneously laser shock peening the object at the first and second sides thereof using laser beams having different pulse lengths to respectively form first and second laser shock peened surfaces on the first and second sides of the object, respectively.
In a preferred form, the first and second laser shock peened surfaces are disposed generally opposite one another.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form at least one set of simultaneously formed laser shock peened surfaces each disposed at a different one of the first and second sides of the object. The respective laser shock peened surfaces of at least one respective set of simultaneously formed laser shock peened surfaces are respectively formed using laser beams having different pulse lengths.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of generally opposing shockwaves to develop simultaneously at different ones of the first and second sides of the object and to subsequently experience different rates of attenuation during propagation within the object.
In a preferred form, the laser shock processor is operated to simultaneously laser shock peen the object at the first and second sides thereof with laser beams having different pulse lengths.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form a plurality of laser shock peened surfaces. The plurality of laser shock peened surfaces includes at least one set of laterally offset, simultaneously formed laser shock peened surfaces each disposed at a different one of the first and second sides of the object.
In a preferred form, the lateral offset relationship is sufficient to enable associated shockwaves induced by laser shock peening to encounter one another within the object.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form a plurality of laser shock peened surfaces. The plurality of laser shock peened surfaces includes at least one set of laterally offset, simultaneously formed laser shock peened surfaces each disposed at a different one of the first and second sides of the object. Each laser shock peened surface is associated with a respective shockwave induced by laser shock peening. Additionally, the respective shockwaves which are associated with at least one selective set of laterally offset, simultaneously formed laser shock peened surfaces encounter one another within the object.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side generally opposing one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of laterally offset shockwaves to develop simultaneously at different ones of the first and second sides of the object and subsequently interact with one another within the object.
In one form, the laser shock processor is operated to simultaneously form a set of laterally offset laser shock processed surfaces on different ones of the first and second sides of the object.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves simultaneously laser shock peening the object at the first and second sides thereof to form first and second laser shock peened surfaces at the first and second sides of the object, respectively, wherein the first and second laser shock peened surfaces have a lateral displacement therebetween.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side generally opposing one another. The method involves operating a laser shock processing apparatus to laser shock process the object in a manner sufficient to produce at least one zone of compressive residual stress in the object that is characterized by an asymmetrical stress distribution profile relative to a reference plane.
In one form, the laser shock processing apparatus is operated to laser shock peen the object at the first and second sides thereof at different times to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively.
In another form, the laser shock processing apparatus is operated to simultaneously laser shock peening the object at the first and second sides thereof using laser beams having different pulse lengths to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively.
In another form, the laser shock processing apparatus is operated to simultaneously laser shock peening the object at the first and second sides thereof to form laterally offset first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively.
The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side generally opposing one another. The method involves causing the formation in the object of at least one asymmetrical compressive residual stress distribution profile imparted by suitable laser shock processing of the object. Each asymmetrical compressive residual stress distribution profile appears generally along a respective thickness dimension of the object.
In one form, the object is laser shock peened at the first and second sides thereof at different times to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively.
In another form, the object is simultaneously laser shock peened at the first and second sides thereof using laser beams having different pulse lengths to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively.
In another form, the object is simultaneously laser shock peened at the first and second sides thereof to form laterally offset first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively.
One advantage of the invention is that the laser shock peening process can increase the penetration depth of compressive residual stress formed below a laser shock peened surface beyond that available from a single pulse.
Another advantage of the invention is that the laser shock peening process can be used to tailor the sub-surface residual stress profile developed through the thickness of a thin section.
Another advantage of the invention is that asymmetry can be introduced into the stress distribution profile by simultaneously laser shock peening both sides of the thin section in the manner described herein, thereby allowing modification of the profile.
Another advantage of the invention is the availability of better control of the depth of the residual stress below the laser shock peened surface, and the intensity of the interaction of the shockwaves at mid-thickness of the thin section.